A changing distance
Supermoons occur about once annually, and this year, the supermoon is closer than it has been in a little while, Thaller said.
The distance from the Earth to the moon varies along the rocky satellite's elliptical orbit. Perigee differs from month to month, so sometimes the supermoon is a little closer or further away, Thaller said.
"The closest the moon gets can actually vary much as much as the diameter of the Earth," Thaller said. "That seems like a pretty big number, but the moon is actually 30 times the diameter of the Earth away from us. If you line up 30 Earths, that's about the average distance of the moon away, but as it swings a little bit closer to us, that distance can vary."
The sun can be to blame for the difference in distance. In the winter, when the Earth is closest to the sun, a supermoon could be even closer and more stunning, Thaller said. The strength of the sun's gravity pulls both the moon and the Earth towards it slightly, making the moon dip closer to the planet.